Sleep apnea (SA) is a common disorder observed in the practice of sleep medicine and is responsible for more mortality and morbidity than any other sleep disorder. Sleep apnea is characterized by recurrent failures to breathe adequately during sleep (termed apneas or hypopneas) as a result of obstructions in the upper airway.
Apnea is typically defined as a complete cessation of airflow. Hypopnea is typically defined as a reduction in airflow disproportionate to the amount of respiratory effort expended and/or insufficient to meet the individual's metabolic needs. During an apnea or hypopnea—commonly referred to as a respiratory event—oxygen levels in the brain decrease while the carbon dioxide (CO2) levels rise, causing the person sleeping to awaken. The heart beats rapidly and blood pressure rises to levels (up to 300 mm Hg). The brief arousals to breathe are followed by a return to sleep, but the apneas may recur over 60 times per hour in severe cases.
Sleep apnea is a serious, yet treatable health problem for individuals worldwide. Published reports indicate that untreated sleep apnea patients are three to five times more likely to be involved in industrial and motor vehicle accidents that have impaired vigilance and memory. Studies show that more than 15% of men and 5% of women over the age of 30 and up to 30% of men and women over the age of 65 suffer from sleep apnea. Sleep apnea during pregnancy is associated with hypertension and a risk of growth retardation in the fetus. Current estimates reveal that over 90% of individuals with moderate to severe sleep apnea remain undiagnosed.
The current standard for the diagnosis of sleep apnea is called polysomnography (PSG), which is administered and analyzed by a trained technician and reviewed by a Board Certified Sleep Specialist. The limited availability of sleep centers coupled with the high capital expense, in order to add capacity for diagnosis of sleep disorders, has resulted in a growing number of patients awaiting analysis by polysomnography.
A conventional full overnight PSG includes recording of the following signals: electroencephalogram (EEG), sub-mental electromyogram (EMG), electroculogram (EOG), respiratory airflow (oronasal flow monitors), respiratory effort (plethysmography), oxygen saturation (oximetry), electrocardiography (ECG), snoring sounds and body position. These signals are considered the “gold standard” for the diagnosis of sleep disorders in that they offer a relatively complete collection of parameters from which respiratory events may be identified and sleep apnea may be reliably diagnosed. The RR interval, commonly referred to as beats per minute, is derived from the ECG. The body position is normally classified as: right side, left side, supine, prone, or up (e.g., sitting erect). Typically, a microphone is taped over the pharynx and the body position sensor is attached over the sternum of the patient's chest. Each signal provides some information to assist with the visual observation and recognition of the respiratory events.
A collapse of the upper airway is identified when the amplitude of the respiratory airflow and the effort signals decrease by at least 50%, snoring sounds either crescendo or cease, and oxygen desaturation occurs. A respiratory event is confirmed (i.e., desaturation not a result of artifact) by the recognition of an arousal (i.e., the person awakens to breathe), typically identified by an increase in the frequency of the EEG, an increase in the heart rate or changing in snoring patter. The remaining signals assist in determining specific types of respiratory events. For example, the EEG and EOG signals are used to determine if a respiratory event occurred in non-rapid eye movement (NREM) or rapid eye movement (REM) sleep. The position sensor is used to determine if an airway collapse occurs only, or mostly, in just one position (typically supine).
A reduction or absence of airflow at the airway opening defines sleep-disordered breathing. Absent of airflow for 10 seconds in an adult is apnea, and airflow reduced below a certain amount is a hypopnea. Ideally one would measure actual flow with a pneumotachygraph of some sort, but in clinical practice this is impractical, and devices that are comfortable and easy to use are substituted. The most widely used are thermistors which are placed in front of the nose and mouth to detect heating (due to expired gas) and cooling (due to inspired air) of a thermally sensitive resistor. They provide recordings of changes in airflow, but as typically employed are not quantitative instruments. Currently available thermistors are sensitive, but frequently lag or have a delay in response time relative to pressure sensors and pressure transducers. Also, if they touch the skin, they cease being flow sensors. Measurement of end tidal CO, is used in some laboratories to detect expiration to produce both qualitative and quantitative measures of a patient's breath.
An alternative method is to measure changes in pressure in the nasal airway that occur during breathing. This approach provides an excellent reflection of true nasal flow. A simple nasal cannula attached to a pressure transducer can be used to generate a signal that resembles one obtained with a pheumatachygraph. It allows detection of the characteristic plateau of pressure due to inspiratory flow limitation that occurs in subtle obstructive hypopneas.
An obstructive apnea or hypopnea is defined as an absence or reduction in airflow, in spite of continued effort to breathe, due to obstruction in the upper airway. Typical polysomnography includes some recording of respiratory effort. The most accurate measure of the effort is a change in pleural pressure as reflected by an esophageal pressure monitor. Progressively more negative pleural pressure swings, leading to an arousal, have been used to define a “Respiratory Effort Related Arousal” (RERA), the event associated with the so-called “upper Airway Resistance Syndrome”. However the technology of measuring esophageal pressure is uncomfortable and expensive, and rarely used clinically. Most estimates of respiratory effort during polysomnography depend on measures of rib cage and/or abdominal motion. The methods include inductance or impedance plethysmography, or simple strain gages. Properly used and calibrated, any of these devices can provide quantitative estimates of lung volume and abdominal-rib cage paradox. However, calibrating during an overnight recording is very difficult and, as a practical matter, is almost never done. The signals provided by respiratory system motion monitors are typically just qualitative estimates of respiratory effort.
Pressure sensing devices are currently available and used during a sleep diagnostic session to detect changes in respiratory air pressure and/or airflow to confirm whether or not a patient is breathing and to gather other breathing information from the patient. Accurate modeling of the patient's breathing cycle is limited by the use of only pressure sensors as the placement of sensors and system failures can cause false readings or pressure offsets that must be adjusted to properly model the breathing cycle.
Combining pressure sensor measurements with temperature sensor measurements can improve breath monitoring and modeling thereby leading to a more accurate diagnosis and more quickly determine a patient's breathing failure by utilizing temperature monitors directly positioned at the nasal and oral breathing passages of the patient. Additionally, in using a temperature sensor for breath monitoring, it is generally necessary to test the electrical leads and circuit components of the temperature sensing device to insure that all of the electrical leads and components are, in fact, operational and not faulty.
In addition, conventional test circuitry typically is completely separate from the temperature sensing device and this leads to further difficulties such as the test circuitry being either misplaced, lost, having insufficient electrical power, etc., thereby rendering it difficult to test the pressure sensing device prior or during use.